An electrical contact is an element for electric circuitry for mechanically closing and opening an electric path by mechanical open/close operation of a pair of contact points. The electrical contact is utilized in switches, relays and so on. Switches and relays which make use of the electrical contact have an advantage that it can provide an excellent open state having a very large electric resistance since the two electrical contact points are mechanically spaced from each other under the open state. For this reason, such mechanical switches and relays are widely used in all fields including information equipment, industrial machinery, automobiles and home electric appliances, as switching means for opening and closing electric circuits composed of power sources, actuators, sensors, and so on.
FIG. 12 and FIG. 13 show a conventional, mechanically opened/closed electrical contact device X3. The electrical contact device X3 includes a mover 71 and a stator 72.
The mover 71 includes a conductor strip 73, a contact 74 provided at an end of the conductor strip 73 and a socket 75 attached to the conductor strip 73. A single conductor strip 73 is provided with a single contact 74. The contact 74 is made of a conductor. The socket 75 is made of resin. The conductor strip 73 has another end to which a lead 76 made of braided copper wire for example is attached mechanically and electrically. The lead 76 is electrically connected with an unillustrated circuit. A pin 77 is inserted through the socket 75, and the mover 71 can swing around the pin 77. The pin 77 is fixed to a predetermined case (not illustrated) which encloses the electrical contact device X3. Pivotal movement of the mover 71 is achieved by a predetermined drive mechanism (not illustrated) which includes an exciting coil for example.
The stator 72 includes the conductor strip 78 and a contact 79 which is made of a conductor. The conductor strip 78 is electrically connected with an unillustrated circuit. The contact 79 is placed on a pivotal path of the contact 73 in the pivotal movement of the mover 71.
In the electrical contact device X3 constructed as the above, assume that a predetermined voltage is applied between the contact 74 and the contact 79. When the mover 71 pivots toward the stator 72 as shown in FIG. 13, bringing the contact 74 into contact with the contact 79, the electric current flows, for example, from the conductor strip 78 through the contact 79, the contact 74, and the conductor strip 73, to the lead 76. Thereafter, when the mover 71 pivots away from the stator 72 as shown in FIG. 12, moving the contact 74 away from the contact 79, the current flow stops. In this way, the electrical contact device X3 connects and disconnects the electric path.
In the field of electrical contact technology, it is known that arcing occurs between a pair of contacts if the contacts are operated into an open state while an electric current is flowing through the closed contacts at a rate exceeding a threshold value (minimum discharge current), or while an electric potential difference is present between the contacts at a rate exceeding a threshold value (minimum discharge voltage). Assume for example, that a closed pair of contacts is to be opened while an electric current which exceeds the threshold value is flowing. As the contacts are being opened, the touching area of the contacts decreases gradually, causing the current to pass through the contacts in an increasingly concentrated manner. As the concentration of the current increases, the temperature of the contacts increases, and surfaces of the contacts melt. Because of this, even after the contacts have been opened, the molten contact material keeps the contacts connected with each other for a period of time while the distance between the two contacts are not large enough. In other words, a bridge is formed between the contacts. From the bridge comes out vapor of the metal, which serves as a medium for arc discharge. The arc discharge develops into a phase where arcing is transmitted by ambient gas, and eventually ceases when the contacts have been spaced from each other by a sufficient distance. This is how arc discharge develops when contacts are opened. A similar mechanism may cause arc discharge when electrical contacts are being closed, because the electrical contacts repeat an intermittent open/close action (bounce) as they are being closed.
FIG. 14 is a graph as an example, which shows dependency of arc discharge probability on electric current between contacts. The graph plots arc discharge probability values when a pair of gold contacts were contacted with each other under a predetermined pressure (10 mN, 100 mN, or 200 mN) and the contacts were opened while a 36 volts was applied between the two. The electrical contacts were connected with a 36-volt constant-voltage power source, with a resistor placed in series. By varying the resistance of the resistor, the electric current flowing through the contacts was varied. The substantial area of contact between the two contacts was believed to be not greater than a few tens of square micrometers. The graph's horizontal axis represents the current which flew through the contacts whereas the vertical axis represents arc discharge probability. Under any closing pressure, arc discharge probability shows about 100% once the applied current reaches or exceeds 0.6 A. On the other hand, when the applied current is 0.1 A or less, arc discharge probability is generally 0%. More details about this graph can be obtained from Yu. Yonezawa, et al. (Japanese Journal of Applied Physics, Japanese Society of Applied Physics, July 2002, Vol. 41, Part 1, No. 7A, pp4760–4765).
From the graph in FIG. 14, it is understood that a minimum discharge current (minimum arc current) Imin which triggers arc discharge is somewhere between 0.1 A and 0.6 A. The minimum discharge current Imin is known to be dependent upon the material species. Likewise, there is a minimum voltage (minimum arc voltage) Vmin necessary for causing arc discharge, which is also known to be dependent upon the material species. For gold contacts, it is reported that the minimum discharge current Imin is 0.38 A, and the minimum discharge voltage Vmin is 15V. It must be understood however, that Imin and Vmin values obtained from actual measurements are not always the same due to influences from the state of electric charge in the space, conditions of the contact surfaces and so on.
When the electrical contact device X3 is closed, all of the electric current needed by the load circuit (an unillustrated circuit which draws the current) flows through the contact 74 and the contact 79. Therefore, if the current drawn by the load circuit exceeds the minimum discharge current, arc discharge is inevitable between the contact 74 and the contact 79 when the contacts are opened. It is not uncommon that the current drawn by the load circuit exceeds the minimum discharge current of the electrical contact device X3.
Every cycle of arc discharge causes melting, evaporation and re-solidification of the material which constitutes the contacts 74, 79, resulting in erosion and transfer of the contact material as well as alteration of contact resistance between the contact 74 and the contact 79. For this reason, reliability and lifetime of the electrical contact device X3 tends to decrease with the number of arc discharges occurring between the contact 74 and contact 79. Reduction in reliability and shortening of lifetime are significant when a large current has to be handled by the electrical contact device X3.
In a conventional electrical contact device X3, it is common that in order to achieve sufficiently small contact resistance in the closed state, the contacts 74, 79 are made of low-resistance metals. Typically, a copper base-material is coated with a low-resistance, corrosion-resistant metal (e.g. Au, Ag, Pd and Pt). However, these low-resistance metals have a relatively low melting point, which means that they easily become molten in the heat generated by arc discharge, and erode or transfer. Metals which are not easily melted in the heat generated by arc discharge have a relatively large electric resistance. In the conventional electrical contact device X3 in which lowering the contact resistance is an important goal, it is practically difficult to use metals which have a high melting point.